Automatic volume control circuit



Nov. 8, i938. H. o. PETERSON AUTOMATIC VOLUME CONTROL CIRCUIT FiledSept. 14, 1935 INVENTOR HAROLD o. PETERSON B QM \M4 h m m v lk A 1 5 0-M K 1 7 qniwwk ATTO R N EY Patented Nov. 8, 1938 UNITED STATES PATENTOFFICE Radio Corporation of Delaware America, a corporation ofApplication September 14, 1935, Serial No. 40,568

5 Claims.

My present invention relates in general to automatic gain regulationcircuits adapted for use in connection with signal amplifiers and radioreceivers, and more particularly the invention relates to automaticvolume control circuits utilizing diodes as signal detectors and gaincontrol devices.

Diode rectifiers are often utilized at the present time as signaldemodulators of radio receiving systems. Such .diode devices are, also,suitable for use as signal rectifiers in connection with automaticvolume control circuits in order to maintain a substantially constantsignal amplitude at the demodulator input circuit. In the past a commondiode device has been used both for the signal detection and automaticvolume control rectifier functions. However, many of these past deviceshave been constructed so as torequire the diode anode to be maintainednormally at a negative direct current potential with respect to thecathodes of the signal transmission tubes whose gain was under control.

According to one of the main objects of my present invention, a dioderectifier device is utilized'to demodulate the received signal, andsimultaneously to rectify signal energy for the production of a gaincontrol voltage which is used to decrease the gain of the signalamplifiers as the received signal amplitude increases, the diode anodebeing normally maintained at a potential which does not becomenegative,and which specifically is positive, in direct current potential withrespect to the cathodes of the controlled tubes.

Another important object of the present invention may be said to residein the provision of a'radio receiving system wherein a signaltransmission tube has its normal negative grid bias provided by theconjoint and opposing action of 40 negative and positive direct currentvoltage sources, and wherein the normal negative grid bias is increased,as the rec-eived'signal amplitude increases, by a diode signal rectifierwhose anode is normally maintained positive with respect to 45 thecathode of the controlled tube.

itself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection withthe drawing in which I have indicated diagrammaticallya circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, which shows the presentinvention embodied in a receiver of conventional construction, it willbe observed that the numeral I designates a signal collector device.This device may be the usual grounded antenna, or it may be a loopantenna; dipole or even a radio frequency signal distribution line. Thecollected signals, which signals may be in the broadcast range or any ofthe short wave ranges, are impressed on the tunable input circuit 2 ofthe radio frequency amplifier tube 3.

The tube 3, shown as a screen grid tube of the indirectly heated cathodetype, has its cathode 4 grounded, and it will be understood that thescreen grid and plate of the tube are connected to suitable sources ofpositive direct current potentials. Such sources are not shown, butthose skilled in the art are-fully aware of the fact that these sourcesmay be of the battery type; or they may be embodied in the usual bleederresistor connected across the filter output of a 60 cycle alternatingcurrent rectifier.

' A grounded electrostatic shield 5 is shown disposed between the signalcollector l and the input coil of the tunable circuit 2, and it will beunderstood that the numeral 6 designates a tuning device which is usedto tune the radio receiver through the desired operating frequencyrange. The signal demodulator, or detector, is a triode of theindirectly heated cathode type, which may be, for example, a tube of the37 type, and is designated by the numeral 1. To provide a diode detectordevice the grid and plate of tube 1 are strapped together, and theresulting diode output electrode is connected to the cathode 8 through apath which includes the signal input coil 9, the coil ID, the condenserll, resistor R2 and the visual indicating meter, or milliammeter l2. Thecondenser l3, connected between the high alternating potential side ofcoil 9 and ground, tunes coil 9 to the operating signal frequency whichis to be detected. A heating battery A is connected across the heaterelement 14 of diode 1, and the positive terminal of the heating source Ais connected to the ungrounded side of resistor R2 through the chokecoil l5, which, of course, has finite resistance.

The audio frequency utilizing network, which may comprise one or morestages of audio frequency amplification followed by a reproducer orother signal indicator, has impressed upon it the demodulated signalenergy through a path which includes the audio frequency transformer [6.The primary winding ll of transformer 16 has one side thereof connectedto the junction of coils 9 and ID through a path which includes coil l8.The resistor R6 is connected in shunt with winding H, the condenser (9being connected in series with winding I I and resistor R6. The by-passcondenser 20 connects the resistor side of coil l8 to ground, and theside of resistor R6 adjacent condenser 19 is connected to ground througha path which includes resistor R4 and resistor R5. is connected to thepositive terminal of heating source A through a path which includesresistor R1 and lead 2|. to the frequency of the carrier being rectifiedby the detector, This affords a low impedance path for the carrierfrequency. The resistance Re I 4 letters AVC to designate that this isthe automatic volume control circuit, and the network comprisingresistor R3 and condenser 23 functions to suppress fluctuating currentcomponents in the gain control voltage.

Signal energy is impressed between the signal grid 4' and cathode 4 ofamplifier 3by virtue of the connection of the signal grid 4' to the highalternatingpotential side of input circuit 2, and the signal by passcondenser 3 connected between the low alternating potential side ofinput circuit 2 and ground. The desired signal energy-is transmitted tothe tuned input circuit 9 of the diode demodulator I through the signaltransmission network generally denoted by the numeral 30. Thisnetwork 30may comprise one,

or more, additional stages oftunable radio fre- The receiver may be ofthe superheterodyne.

type. In such a case the network 30 may comprise additional signalfrequency amplifier stages,

followed by the usual first detector and local oscillator circuits forthe production of a desired intermediate, frequency. The first, detectoroutput will be followed by one, or more, stages of intermediatefrequency amplification, A sec-' ond converter can beused to change thefirst I. F. (say 300 kc.) to a second I. F. (say 50 kc), and the latterwill be amplified before detection. In that case the demodulator 1functions at in termediate frequency; Of course, the condenser l3 in thecase where the receiving system is of the superheterodyne type is fixedin value to tune a the input circuit of the detector to the desiredarrangement of the present type may be emp yed.

While the present circuit diagram shows the AVG voltage applied to onlyone of the signal transmission tubes of the receiving system, it will beclearly understood that the gain control bias The junction of resistorsR4 and R5- 4 Coil l0 and condenser I l resonate may be applied to any ofthe additional signal transmission tubes preceding the final signaldemodulator. Thos'eskilled in the. art will, in general, understand theoperation of the automatic volume control arrangement shown herein.Normally,'and in the absence of received signals, the automatic volumecontrol arrangement is not "functioning. When the desired signals arereceived, the AVG network begins to increase the negative biason thesignal grid 4. As the signal amplitude increasesfthe gain control biasincreases, and in this way the gain of the signal transmission tube 3 isdecreased. In other words, duringperiods when no signals are receivedthe gain of the signal transmission tube 3 is a maximum, and thereceiver is in a highly sensitive condition. The cold electrode of thediode detector is maintained normally;that is to say in the absence ofdesired signals, at a positive potential with'respect to the groundedcathode 4'. This positive potential may be, for example, of. the orderof 0.47 volt, and is supplied from the positive terminal of the heatingsource-A. The latter has a voltage of about 6 volts. This heating sourcenot only supplies heating current to the filament of the detector 1,but, also, impresses a potential-across the resistors R1 and R2. The

approximate voltages at the'ungrounded sideof' sistance.

Asa consequence of these potential relations, the AVG action does not,in the absence of desired signals maintain the signal grid 4' highlynegative;' on the other hand, it maintains the signal grid! at arelatively small negative potential with respect to the grounded cathode4, and rather close to zeroidirect current potential, with theresultthatduring periods of tim'e'whenno signals are impressed uponthe collector'Lthe signal amplifiers are in condition to produce maximumamplification of the received signals. This follows by virtue of thefact that the cold electrode of the diode is normally maintained at aposi-'-' tive potential with respect to the grounded oath-'- ode 4.

That is to say, while the junction point of re: sistors R1 and Rs is atapproximately +0.5 volt, the signal grids of the controlled amplifiersare not positive by'this amount, but in fact are at a small negativepotential with respect to the grounded cathodes of the controlled signaltransmission tubes. This is explained by the fact that when the cathodesof the signal transmission tubes are heated to an electron emittingcondition, electrons leave the cathodes at such high velocities thatthey impinge upon the signal control grids adjacent them. This causes acertain currents flowing through the path including resistors R3, R4, R5and then to ground. Normally,

this rectified grid current would maintain thesignal grids of thecontrolled transmission tubes at a negative potential with respecttotheir cathodes in the absence of received signals. This negativepotential is reduced very slightly'b y the opposition of the positivevoltage of 0.5 volt existing at the junction of resistors R1 and B5.

In this condition the control grid bias of these tubes assumes a valuebetween 0.5 and -1.2

volts.- This assumed bias happens to be of a value amount ofrectification action, the rectified grid 7 at which these tubes haveapproximately maximum gain. As received signal strength is increasedirom zero, the junction of resistors R6 and R4 decreases in positivepotential, goes through zero potential, and then increases in negativepotential. The bias of the controlled tubes is practically unchangeduntil the potential of the junction of Re and R4 reaches a negativevalue equal to the negative value assumed by those grids in theno-signal condition. As the signal strength is further increased thegrid bias of the controlled tubes will be the same negative potential asthat of junction Re and R4 since for such increasingly negative values,no current will be drawn by the grids of the controlled tubes. From thispoint on, the gain is progressively decreased as the signal amplitudeincreases. In this way the signal amplitude at the input circuit ofdetector l is maintained substantially uniform in spite of a wide rangeof signal amplitude variation at the collector I. It will be recognizedthat the action described above is a form of delayed AVC.

The junction of resistors R6 and R4 is at positive potential for zerosignal; and is at a negative potential for signals of such strength thatthe direct current voltage drop in R4 is greater than the fixed bias onR5. R3 is normally between ten times and one hundred times as great asR4 and R5. Also R3 is great compared to the internal resistance betweengrid and cathode of the controlled amplifier tubes when the grids areoperated at potentials such that grid current fiows.

For this reason the grids of the controlled stages become less negativeonly very slightly for changes in the potential of the junction of R6and R4 as long as the potential of this junction is such that gridcurrent will flow through R3. As soon as this junction becomessufficiently negative so that no grid current flows through R3 there isno potential drop in R3, and, consequently, from then on to much morenegative Values of the junction it will be found that the grids have thesame potential as the junction of Re and R4.

The voltage supplied at the junction of coil l and R2 could be suppliedby a tap on a bleeder resistance across a rectifier output derivingpower from the main supply. Likewise, the usual alternating currentpower supply bleeder resistor may be used to supply the positivepotential indicated at the junction of R1 and R5.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention,-as set forth in the appended claims.

What I claim is:

1. A radio receiving system of the type including a signal transmissiontube followed by a signal rectifier of the diode type, means forestablishing a normal negative grid bias for said transmission tube bythe conjoint and opposing action of'indep'endent negative and positivedirect current voltage sources, said negative voltage source comprisinga circuit in the grid circuit of the transmission tube and through whichgrid current flows in the absence of signals of a desired amplitude,means for preventing the rectifier anode from assuming a direct currentpotential which is negative with respect to the cathode of saidtransmission tube, and means for applying the direct current voltageoutput of said rectifier to the grid of said transmission tube in asense such that the said normal negative grid bias is increased as thereceived signal amplitude increases.

2. A radio receiving system including a controlled electronictransmission stage, a diode detector with its anode and cathode bothnormally at positive potentials with respect to the cathode of thecontrolled electronic transmission stage, a load circuit and networkconnected to the diode so as to obtain modulation frequency output asWell as direct current output, the direct current output being connectedin series with a fixed source of positive direct current potential and aseries resistor between the grid of the controlled electronictransmission stage and the cathode of the latter whereby the grid ofsaid stage will normally assume a substantially constant potentialnegative with respect to the oathode of said stage until the receivedsignal reaches such a value that the negative direct current output ofthe diode exceeds the above mentioned series fixed source of positivepotential.

3. In a signaling system having at least one signal transmission tube, adiode, means for normally establishing the anode and cathode of thediode at positive potentials with respect to the transmission tubecathode, means for deriving from'the rectified output of the diode adirect current voltage, means including a source of substantiallyconstant positive potential and a source of negative potential incircuit with the transmission tube grid circuit for establishing anormal negative bias on said transmission tube, said direct voltagederiving means being connected to the transmission tube grid circuit insuch a manner that said normal negative bias value is increased when themagnitude of the derived voltage exceeds that of the positive potential.

4. In combination with a signal transmission tube, means includingopposed positive and negative direct current voltage sources forestablishing a normal negative bias on the signal input electrode ofsaid transmission tube with respect to the cathode thereof, said sourcesbeing independent, and the negative source comprising a network includedin circuit with said input electrode and through which network currentfiows for signals below a predetermined amplitude, a diode signalrectifier, means electrically associated with thefirst said means forpreventing the diode anode from assuming a negative potential withrespect to the transmission tube cathode, and additional means forapplying the direct current voltage output of said rectifier to theinput electrode of said transmission tube in a sense to increase saidnormal negative bias as signal amplitude increases.

5. In combination with a signal amplifier of a radio receiving tube, adiode rectifier for the output of said signal amplifier, means coupledto said rectifier for utilizing the modulation component of therectifier output, means for deriving a direct current voltage from therectified output of said rectifier, means for normally maintaining thediode anode and cathode at a direct current potential which is positivewith respect to the oathode of said amplifier, and additional means forapplying said direct current voltage output of the rectifier to theinput electrodes of said amplifier in a sense to decrease the gain ofthe amplifier as the received signal amplitude increases.

HAROLD O. PETERSON.

